1. Technical Field
The present invention relates to an optical glass suitable for a high-precision press molding at a relatively low temperature.
2. Related Art
In recent years, digital cameras or mobile phones equipped with a camera capable of taking image information by using an image pickup device such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) have spread-rapidly. In particular, recently, as image pickup devices having a large number of pixels are developed in order to achieve high image quality, a demand for high optical performance in an imaging lens has become stronger. On the other hand, a demand for a decrease in size has also become stronger.
In order to cope with the demands, usually, molded lenses made of glass press-molded by a mold having a precisely accurate size are employed as the imaging lens. By using the press molding as compared with grinding processing, it is possible to manufacture easily and efficiently an optical lens having an aspheric surface and an optical lens having a micro size.
However, since such press molding is performed at a high temperature equal to or more than a deformation temperature of an optical glass as a raw material, high durability is required for the mold to which physical load such as heat or stress is applied. As the deformation temperature of the optical glass increases, the physical load applied to the mold increases. Accordingly, in order to increase lifetime of the mold, suppressing the deformation temperature of the optical glass as low as possible is required.
On the other hand, in order to decrease a size of the imaging lens and increase an angle of view of the imaging lens, an optical glass having a high refractive index or a low dispersivity is also strongly demanded.
There are developed several optical glasses having a high refractive index, a low dispersivity, and a relatively low deformation temperature (and a glass transition temperature) (for example, refer to Patent Document 1 (JP-A-2005-263570 corresponding to US-A-2005/0204776), Patent Document 2 (JP-A-2005-97102 corresponding to US-A-2005/0049135), Patent Document 3 (JP-A-2006-16293 corresponding to US-A-2005/0272589, US-A-2009/0149307 and US-A-2008/0220961), Patent Document 4 (JP-A-2006-16295 corresponding to US-A-2005/0272589, US-A-2009/0149307 and US-A-2008/0220961), Patent Document 5 (JP-A-2008-105863), or Patent Document 6 (JP-A-2004-175632 corresponding to US-A-2004/0220041)). The optical glasses disclosed in Patent Documents 1 to 6 includes silicon oxide (SiO2), boron oxide (B2O3), lanthanum oxide (La2O3), gadolinium oxide (Gd2O3), and zinc oxide (ZnO).
However, recently, a decrease in size of the imaging lens and an increase in performance are remarkable, and thus there have been additional demands for an increase in refractive index of the optical glass, a decrease in dispersivity of the optical glass, and processability.